It is not uncommon for the spindles of modern rotating machine tools such as drilling or milling machines to operate at very high speeds of 20,000 RPM and more. At these speeds, powerful centrifugal forces are generated with even a slight imbalance. These centrifugal forces not only strain the spindle bearings of the machine tool, but also shorten the service life of the tool and may also impair machining results. The tool holder (with or without a clamped tool) is therefore balanced in a balancing machine before being used in the machine tool. A typical representative of such a balancing machine is described in patent application WO 00/45983.
The central component of balancing machines of the type known from WO 00/45983 is the spindle to which the component to be balanced is fastened, which then brings the component to be balanced to the balancing speed. The spindle travels in a spindle holder and together with it, constitutes the so-called spindle unit. In modern balancing machines, the imbalance is determined directly in the spindle unit—the instantaneous forces occurring in a particular direction in the spindle unit are detected by means of a suitable sensor and constitute a measure for the magnitude and location of the imbalance.
In order to be able to generate the corresponding measurement signal as a function of the respective imbalance, the spindle unit in balancing machines of this kind is suspended in a special oscillating bearing. In the embodiment of WO 00/45983 described below as a representative example, this oscillating bearing is composed of two leaf springs embodied in the form of thin sheets of spring steel. Each of these leaf springs is fastened with its one end to the machine frame and with its other end to the outer circumference of the spindle unit. The two leaf springs are situated spaced apart from each other in a vertical plane extending radially to the spindle rotation axis. In this plane, they resist the action of tensile, compressive, and shear forces, but behave in an essentially flexible fashion in response to force components oriented perpendicular to this plane. The upper of the two leaf springs (in the above-mentioned vertical plane) is loaded with tensile force in the horizontal direction and shear force in the vertical direction while the lower leaf spring is loaded with compressive force in the horizontal direction and with shear force in the vertical direction. In this way, the spindle unit is secured in an elastically cantilevered fashion, thus permitting the occurrence and detection of those imbalance-induced movements of the spindle unit that permit conclusions to be drawn about the position and magnitude of the imbalance.
As shown in FIG. 3 of WO 00/45983, the rotating imbalance force sets the spindle unit essentially into a horizontally oriented pendulum motion around the oscillating bearing mounted at the 12 o'clock position; the pendulum motion has only a very small amplitude that meets practically no resistance from the leaf springs. This results in an actuation of the force sensor protruding like a finger from the machine pedestal at the 3 o'clock position. However, the two leaf springs behave rigidly in the vertical direction so that the spindle unit executes little or no relevant movements in the vertical direction under the influence of the imbalance-induced forces and also executes little or no relevant flexing in the vertical direction under the influence of a specimen that does not exceed the rated load.
The known embodiments—and in particular, embodiments that follow the embodiment principle or suspension principle known from WO 00/45983—have the problem that the bearing of the spindle unit is susceptible to shocks and overloading, i.e., can be damaged or at least disadvantageously influenced by a careless insertion of a specimen or by the insertion of a specimen that is too heavy.
The object of the invention is to eliminate this problem and to disclose a more rugged device that functions properly, even in cases involving heavy specimens.